The present invention relates to a track-following control system for a magnetic disk drive and, more particularly, to a track-following control system using a sector servo method for a high track density magnetic disk drive.
Recently, a means for increasing the density of tracks concentrically formed on a magnetic disk, for instance a floppy disk, has been employed in order to increase the recording capacity of the floppy disk as a recording medium of a floppy disk drive (FDD).
Information is recorded in and read out from the floppy disk by a magnetic head which can freely move above the surface of the floppy disk. Generally, the magnetic head is moved or seeks from a track on or from which information is currently being recorded or read out (to be referred to as a current track) to a track on or from which information is to be recorded or read out (to be referred to as a target track) under the control of a head positioning system using a servo control technique, and the magnetic head is positioned on the target track.
In the magnetic head positioning system using servo control, when the magnetic head is to be moved from the current track to the target track and is to be positioned on it, the magnetic head is roughly moved close to the target track without using the servo control (to be referred to as coarse positioning), and is then finely controlled by the servo control to follow the target track.
As the track density is increased, it is difficult to perform correct track following with a conventional open loop head positioning method. Therefore, a track-following system of a sector servo method using a closed loop feedback technique has been recently developed and used in practice.
According to the sector servo method, the tracks are divided into a plurality of sectors. A track position signal indicating the track position or servo information is recorded at part of each sector, i.e., the beginning of each sector. The servo information is read by the magnetic head. A position error signal representing a deviation between the position of the magnetic head and the track position, i.e., a position error, is obtained by using the servo information. The magnetic head is driven by a servo mechanism to minimize the position error in accordance with the position error signal, and follows the track. More specifically, in the track-following control system using the sector servo method, the magnetic head is held at a position determined on the basis of a previous servo information until a next adjacent servo information is obtained.
The number of bytes of data per sector must be variable in accordance with the operating system (OS) of the information processing system from the viewpoint of information processing. Therefore, when the number of bytes of data per sector is increased, the number of sectors is decreased. As a result, the volume of servo information is decreased and, accordingly, the number of position error signals obtained as described above is also decreased. In order to increase the data recording capacity of the floppy disk, a ratio of the data recording area with respect to the entire floppy disk area must be increased. The number of position error signals is limited from this viewpoint as well.
When the number of position error signals is decreased in the track-following control system using the sector servo method, the following problems arise because of the nature of the floppy disk.
The floppy disk has two modes of eccentricity, i.e., track distortion. The first mode eccentricity is caused by floppy disk exchange. The second mode eccentricity is caused by the environmental changes, such as a change in temperature and humidity. Of these two eccentricities, the second mode eccentricity is particularly important in design of a track-following control system.
FIG. 1 is a view for explaining a track distortion formed on a floppy disk by the environmental changes.
Referring to FIG. 1, a solid line 2a denotes a normal circular track without deformation; a long-and-short-dashed line 2b and a broken line 2c, represent deformed tracks, respectively.
Sectors 1--1 to 1--8 are formed to divide the circular track 2a into 8 equal portions. Servo information SI1 to servo information SI8 are recorded at the beginning of the sectors 1--1 to 1--8, respectively.
Since the floppy disk is made of a polymer material, it greatly expands or contracts by a change in temperature and humidity. The floppy disk has so-called anisotropy wherein the coefficient of expansion differs in the vertical and horizontal directions. Therefore, as shown in FIG. 1, the circular track 2a in the normal condition is deformed to elliptic tracks 2b and 2c when the floppy disk expands and contracts, respectively.
FIG. 2 is a developed view of FIG. 1. In FIG. 2, the angle of rotation of the floppy disk rotating in the direction of the arrow in FIG. 1 is measured with reference to the x-axis and plotted as the axis of abscissa, and the distance between the intersection of the x-axis and the track and a disk center C is plotted along the axis of ordinate to indicate the position of the track. The scale marks 1 to 8 on the axis of abscissa indicate servo information.
As shown in FIG. 2, the tracks 2b and 2c obtained when the floppy disk expands and contracts, respectively, form waveforms of 2 periods per revolution of the floppy disk.
FIG. 3 is a view for explaining the problems occurring in the track-following operation for the deformed track 2b when the floppy disk expands.
A straight line 3 indicates the position or track of the magnetic head on the floppy disk immediately before start of the track-following mode when the magnetic head has been coarsely positioned on the track 2b by coarse positioning as described above. The long-and-two-short-dashed lines 4 and 4' drawn on two sides of the straight line 3 at equal distances from it define a position error detectable range R which is predetermined in the track positioning system. A position error regarding a track falling outside the range R defined by the straight lines 4 and 4' cannot be detected.
In FIG. 3, since the servo information SI2, the servo information SI4, the servo information SI6, and the servo information SI8 of the even-numbered sectors on the track 2b fall within the magnetic head position error detectable range R, they can be detected. However, since the servo information SI1, the servo information SI3, the servo information SI5, and the servo information SI7 of the odd-numbered sectors on the track 2b fall outside the position error detectable range R, they cannot be detected. In other words, despite that the magnetic head is positioned at an average position of a target track, it cannot follow the target track by the servo control since the track has a distortion due to the anisotropy in expansion coefficient of the floppy disk. As a result, data information cannot be regenerated.
This applies to contraction of the floppy disk because of the following reasons. When an FDD is manufactured, its material and structural design are appropriately selected such that a change in position of the magnetic head caused by a change in temperature and humidity compensates for a change in position of the track caused by expansion or contraction of the floppy disk.